Long spans in optical communication networks (e.g. submarine links) require Raman amplification due to its better noise characteristics. The most demanding connections, such as those to distant islands, long festoons, and but skipping spans, require, in addition to the usual counter-propagating Raman pumping scheme, the use of the co-propagating (also referred to a forward pumping) Raman scheme. Higher order Raman pumping schemes can also be used for further performance enhancement.
The co-propagating Raman amplification scheme is the most critical since the signal power is at its highest level in the optical link and gets amplified by the Raman pump, so nonlinear effects can degrade performance if an inappropriate channel signal power is set. The co-propagating Raman amplification scheme is also affected by the ‘gain saturation’ phenomenon where the gain of the Raman amplifier can be reduced due to pump depletion induced by high signal power amplification; the higher the signal power, the higher the gain reduction due to pump depletion. As a result, finding the optimal channel signal power to launch an optical signal at in an optical communication network configured for co-propagating Raman amplification is a difficult task and usually needs to be done by expert personnel in the field, with many try and see steps. In addition, when the channel count changes, for example due to network upgrades, the channel launch power needs to be re-optimized because gain saturation couples the gain of the Raman amplifier with the channel count.
Existing Raman amplification networks are operated to keep the gain of the Raman amplifier constant by adjusting the Raman pump power when the channel count changes. An example of this approach is reported in P. Kim and N. Park “Semi-Analytic gain control algorithm for the fiber Raman amplifier under dynamic channel reconfiguration”, Optical Fiber Communication Conference, 6-11 Mar. 2005, OTuN5.